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VI. The Prospects of Improvements in Other Plants
OLIVER E. NELSON
yet known whether the accession grown in another year would again
have a high lysine content.
Assuming that one could detect mutations that suppress prolamine
synthesis and significantly change the prolamine: glutelin ratio in any of
the cereals rich in alcohol-soluble proteins, the results might not be as
striking as with the o1 and j12 mutations in maize as Mertz (1969) has
pointed out. The prolamines of cereals can be divided into 3 groups on
the basis of their amino acid composition: the first group contains the
gliadin of wheat, hordein of barley, and secalin of rye; the second contains the zein of maize, panicin of millet; the third contains the avenin of
oats (Mosse, 1968). The prolamines of the second group contain very low
amounts of lysine. To this group can be added kafirin of sorghum with a
lysine content of less than 0.2 g. per I00 g. of protein. The consequence of
suppressing prolamine synthesis with consequent compensatory synthesis of other protein fractions would be expected to be greater in terms
of lysine content for maize, millet, and sorghum than for cereals of the
first group, where the lysine content of the prolamine fraction is ca. 1 g.
per 100 g. of protein.
The possibility of raising the methionine content in the proteins of the
legumes deserves careful study since methionine is the limiting amino
acid for all legumes. For obvious reasons, the proteins of the soybean
seed have been intensively investigated. As in the other leguminous
seeds, the bulk of the seed protein is globulin in nature and was once
thought to be a homogeneous protein (glycinin). Ultracentrifugal studies
have revealed that 4 components (2, 7, 1 1 , and 15 S) are present (Naismith, 1955). Wolf and Sly ( 1 967) have shown that other methods of
fractionation will separate the components. Roberts and Briggs ( 1 965)
reported that the 7 S component that comprises 30 percent of the total
protein has an extremely low methionine content-0.19 g. per 100 g. of
protein. For comparison, the entire globulin fraction has a methionine
content of 1.4 g. per 100 g. of protein. The 7 S fraction also differs appreciably from the total in its content of threonine and glycine. If the
synthesis of the 7 S fraction could be blocked or suppressed genetically,
and compensatory synthesis of the other fractions resulted, the methionine content would be raised substantially. Wolf et al. (1961) have reported that the relative proportions of the 7 S and 1 1 S fractions were
quite different in Clark soybeans grown in Illinois and Hakuhou No. 1
soybeans grown in Japan. In this instance, it is not clear whether variety,
location, or both are responsible. The possibility that lines with a low
quantity of the 7 S fraction exist, or could be induced, should be investigated.
In this review, the principal concern has been the enhancement of
GENETIC MODIFICATION OF PROTEIN QUALITY IN PLANTS
biological value of cereal and legume seed proteins. It appears that this
is most likely to be achieved by altering the relative proportions of
storage proteins that have different amino acid compositions, but changes
in nutritional value may arise through other circumstances. In the cereals,
the proteins of the germ are much superior to those of the endosperm in
nutritive value. Tables I 1 and I11 demonstrate this point for maize. If a
greater proportion of the protein were germ protein, the biological value
of the protein would be increased. I t is possible that the differences in
lysine content in different races of Mexican maize (Tell0 et al., 1965) may
be explained by varying germ :endosperm ratios.
Potato tubers from different varieties may have a 2-fold range in the
content of such essential amino acids as lysine and methionine (Nehring
and Schwerdtfeger, 1957). Reissig ( 1958) has shown that potato tubers
have a substantial portion of their total nitrogen as nonprotein nitrogen
(free amino acids). The true protein fraction as distinguished from crude
protein (the total nitrogen content x 6.25) has a good amino acid balance.
The proportion of nonprotein nitrogen varies in different varieties (40-54
percent). The content of essential amino acids expressed as an EAA
index (Oser, 195 1 ) was much higher in the protein fraction (EAA index
83 to 89 percent) than in the nonprotein fraction (EAA index 31 to 43
percent). The protein content was highly correlated with the length of the
growing season- the later the variety, the greater the percentage of
nitrogen that was present in the protein fraction and the higher the EAA
index. Within maturity groups, there were still differences between
varieties as to the percentage of nitrogen present as true protein. The biological value of potatoes could be raised by selection for lines that could
synthesize larger quantities of protein within a given maturity season.
Toxic substances present in seeds can be important deterrents to the
use of their proteins. Liener (1966) has reviewed the subject of both
proteinaceous and nonproteinaceous substances in seeds that present
problems because of their toxicity. The legumes as a group contain an
array of antinutritional factors - trypsin inhibitors, hemagglutinins, and
goitrogenic substances. Since these compounds can be destroyed by the
proper heat treatments, no program to lower their concentration in the
seeds seems justified. An exception is Lathyrus sativus, cultivated on 5
million acres in India. Consumption of this legume can result in permanent paralysis apparently caused by P-N-oxalyl-a,P-diaminopropionic
acid. Although the toxic substance can be extracted by thorough cooking,
the cooking water being discarded, or by soaking in cold water and steeping in hot water (Mohan er al., 1966), it would obviously be desirable to
identify strains lacking the toxin or having a very low content.
The use of meals remaining after oil is extracted from the seeds of a
OLIVER E. NELSON
number of cruciferous plants is limited by the presence of thioglycosides
that are enzymatically hydrolyzed to yield goitrogenic isothiocyanates.
As the hydrolytic enzymes are present in the meal, the meal can be
moistened to enable hydrolysis to occur. The isothiocyanates can then
be removed by steam distillation. Lines of Brassica carrzpestris much
lower in thioglycosides have been identified (Josefsson and Appelqvist,
1969). Possibly such strains can be used in breeding programs to achieve
varieties sufficiently low in thioglycosides to be used without hydrolysis
In order to utilize the proteins of cottonseed meal in animal or human
diets, it is necessary to remove the toxic pigment gossypol by solvent
extraction. Strains of cotton low in gossypol can be selected (Rhyne
et al., 1959). Meal from these strains is equal in nutritive value to that of
solvent-extracted commercial meal (F. H. Smith et al., 1961). Gossypol
is eliminated altogether in genotypes lacking the pigment glands in which
gossypol is produced (McMichael, 1960). If mutant plants produce fibers
equal to normal plants in quality, the introduction of the mutation into all
commercial varieties would make the use of unextracted cottonseed meal
One other possibility deserves a brief mention. The cereal grains contain only low quantities of free amino acids. The production of any amino
acid is evidently regulated to correspond closely to the demand for it in
protein synthesis and other reactions. The mechanism(s) of such regulation has not been intensively investigated in higher plants, but much is
known about the regulation of amino acid synthesis in microorganisms
where one or more enzymatically mediated reactions may be key reactions from a regulatory standpoint. A mutation may cause the loss of
sensitivity to the usual signals repressing enzyme synthesis or a loss of
sensitivity by the enzyme to the usual factors restricting its activity
(Sheppard, 1964; Calvo and Calvo, 1967). In either case, the effect could
be an oversynthesis of a particular amino acid in terms of the requirements for protein synthesis and hence some quantity of that amino acid
present as the free amino acid. No mutation of this type has ever been
identified in-higher plants, but the possibility should be considered.
Serious attention should be given to the identification and utilization of
mutant genes that raise the concentration of the limiting amino acids in
both cereals and legumes that are important sources of protein for
humans and livestock. The improvement of the nutritional quality of
GENETIC MODIFICATION OF PROTEIN QUALITY I N PLANTS
traditional foods has many advantages, particularly in developing nations
where it may be difficult to reach large segments of the population with
The factors tending to enforce a relatively uniform amino acid composition for a species have been emphasized as an essential background for
those contemplating research in improving protein quality. Considering
the restrictions on change in amino acid composition, there still exist
opportunities to effect improvement in all the cereals where large quantities of alcohol-soluble proteins are synthesized. The possibility of improvement in the legumes appears less good, although a recent intriguing
report of heterogeniety in methionine content in different fractions of
soybean storage proteins may indicate that progress could also be
From theoretical considerations involving the genetic control of protein synthesis, it is probable that the most probable avenue to important
changes in amino acid composition involves changing the relative proportions of metabolically inert storage proteins that have quite different
amino acid compositions. This view has been reinforced by a study of the
effects of the o2 and f12 mutations in maize. These mutations enhance
markedly the nutritional value of maize seed proteins. Other possibilities
of effectively changing the amino acid composition of cereal grains
toward improved nutritional quality involve increasing the germ:endosperm ratio or mutations that relieve the constraints ordinarily regulating
the amount of an essential amino acid synthesized.
The supply of readily available plant protein may also be increased by
the selection of strains lacking or low in toxic substances that must be
destroyed or extracted before the seed proteins of cotton and many
species of the Cruciferae can be utilized.
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THE EXTRACTION, CHARACTERIZATION, A N D
SIGNIFICANCE OF SOIL POLYSACCHARIDES
G. D. Swincer, J. M. Oades, and D. J. Greenland
Waite Agricultural Research Institute, University of Adelaide, South Australia
I. Soil Carbohydrates
11. The Significance of
111. Studies on Soil Polysaccharides .............................................................
B. Extraction of Polysaccharides from Soils
C. Purification of Soil Polysaccharides ..................................................
D. Fractionation of “Purified” Soil Polysaccharides ................................
E. Properties of “Purified” Soil Polysaccharides
F. The Origin, Synthesis, and Decomposition of
IV. Methods for the Analysis of Complex Polysaccharide Materials ..................
A. Introduction .................................................................................
B. Analytical Methods .......................................................................
C. Separation Methods ...
V. Summary and Conclusions
I. Soil Carbohydrates
Although much is known about the nature and function of many polysaccharides synthesized by individual organisms, there is little information relating to the polysaccharides produced in an environment such as
the soil which in a unique way brings together a great variety of biological
forms. The comparative neglect of soil polysaccharides is perhaps surprising when it is realized that soils not only support the majority of
higher plants but are the chief habitat for microorganisms. The amount of
polysaccharide material added to soils as plant residues or synthesized
in them by microorganisms must be enormous. Evidence that at least
some of the polysaccharides produced in soils are capable of improving
the stability of soil aggregates and therefore of encouraging the maintenance of an agriculturally favorable structure has provided the main
stimulus for the study of these compounds. The slowness of progress can
be attributed largely to the technological difficulties inherent in the
study of any system as complex as the soil.
G . D. SWINCER, J . M. OADES, AND D. J . GREENLAND
The carbohydrates of soil are composed of a wide range of monosaccharides. Hexoses, pentoses, various deoxy and 0-methyl sugars,
uronic acids, and amino sugars have been identified in hydrolyzates of
numerous soils and soil extracts (Mehta et al., 196 1 ; Gupta, 1967). The
presence of such a variety of components makes precise measurement of
total soil carbohydrates very difficult, and this difficulty is aggravated by
the low stability of most of the carbohydrate monomers under conditions
so far found necessary for their release from polymeric compounds. However, the quantitative determinations that have been made indicate that
carbohydrates constitute between 5 percent and 25 percent of the soil
Free monosaccharides constitute less than 1 percent of the soil carbohydrates, and extracted polysaccharides have rarely accounted for more
than 20 percent (Mehta et al., 1961; Gupta, 1967). Approximately another 10 percent may consist of cellulose (Gupta and Sowden, 1964).
More recently, techniques have been developed that enable almost complete extraction of carbohydrates from soil (Swincer et al., 1968a,b). The
composition of carbohydrates removed by vigorous extraction procedures
is similar to that of materials removed by less efficient methods, and the
reason for differences in the ease of extraction of polysaccharides from
different soils would appear to be physical rather than chemical.
I I . The Significance of Soil Polysaccharides
The main stimulus for the study of soil polysaccharides has been the
repeated indications of their influence on soil physical conditions. The
polysaccharides undoubtedly also affect other soil properties such as
cation exchange capacity (due to the uronic acid units), the retention of
anions (due to amino groups, but only in acidic soils), carbon metabolism,
biological activity (e.g., by acting as an energy source for heterotrophs),
and the complexing of metals.
Interest in the relationship between the physical properties and the
polysaccharide components of soils was aroused by several reports which
indicated that microbially produced gums could bind soil particles into
stable aggregates (Winogradsky, 1929; McCalla, 1943, 1945; J . P.
Martin, 1945a, 1946; Geoghegan and Brian, 1946, 1948; Haworth et al.,
1946; Swaby, 1949). More recent work (Clapp et al., 1962; Harris et al.,
1963; J. P. Martin and Richards, 1963; J . P. Martin et al., 1965) has confirmed the earlier observations. The presence in the soil of organisms that
produce aggregate-stabilizing gums when cultivated in the laboratory (J. P.
Martin, 1945a,b; Forsyth and Webley, 1949; Forsyth, 1954; Bernier,